ePAPR states that any non-boot CPU will come in "disabled" state. So we should
not consider a "disabled" cpu as a CPU we have to ignore, and we should use
them if they provide a "enable-method".
While I'm there, support "ok" as well as "okay", while ePAPR only accepts
"okay", linux accepts "ok" too so we can expect it to be used.

Reviewed by: andrew (partially)